Abstract
• Powder delivery systems for directed energy deposition processes can exhibit oscillatory variation in mass flow rates, matching the characteristic speed of individual hopper components. • Flow rate variation can be observed at the nozzle exit, both by offline weight measurement and by imaging the powder flow. • Flow rate variation is observed to have a measurable effect on melt track height, with the potential to cause build quality issues. • Melt pool imaging shows a correlating pattern, indicating that flow rate variation can affect melt pool image brightness, and may influence the behaviour of image based control approaches. Powder flow rate is a key parameter in Directed Energy Deposition (DED) processes. During a typical build, if powder flow rate is reduced for just 1 second, 30 mm of melt track is affected. Consequently, even a small variation in powder flow rate can have significant implications on build quality. In this work, the powder flow stability for different types of 316 L steel powders was quantified using a combination of methodologies including offline weight measurements, flow imaging, in-situ build data and coaxial melt pool imaging. Flow rate oscillation was observed, correlated with the periodicity of powder hopper turntable rotation, at sufficient magnitude to cause build quality effects and be identifiable in coaxial melt pool imaging. The implications of flow rate variation on the use of melt pool imaging for closed-loop control are discussed.
Highlights
Directed Energy Deposition (DED) systems use nozzles to focus a mixed stream of metal powder and gas into the melt pool, making it one of the more critical components affecting build quality[1,2]
The primary focus was the frequency of hopper turntable rotation, which is the characteristic of this hopper design most likely to introduce variation
The hopper was loaded with approximately 3 kg of GA316 powder and >200 g of powder was run through the pipework to allow the system to stabilise before collecting data
Summary
Directed Energy Deposition (DED) systems use nozzles to focus a mixed stream of metal powder and gas into the melt pool, making it one of the more critical components affecting build quality[1,2]. Studies have considered how the nozzle design influences the shape of the powder cone, using a range of measurement and modelling techniques to determine the shape and position of the powder focal point compared with the laser focal point[1,2,8]. These have included analysis using line lasers and high speed cameras to understand the shape of the powder flow distribution[9,10,11], which has led to a commercially available system developed by Fraunhofer IWS[12]. These studies have considered snapshot behaviour from a single image exposure rather than how the flow rate and shape of powder distribution varies over time
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